1. Field of the Invention
The present invention relates to a method of purifying an aqueous solution of hydrogen peroxide, particularly to an adsorbent for production of a highly pure aqueous solution of hydrogen peroxide used for washing in the electronic industry and a purification method using the adsorbent.
2. Description of the Prior Art
As integrated circuits are densified, higher purity is demanded for washing solutions to be used in the electronic industry. Among them, an aqueous solution of hydrogen peroxide is used as an important washing solution and is required to have such a quality as a metal impurity concentration of at most 0.1 ppb and a total organic carbon (TOC) concentration of at most 10 ppm.
At present, aqueous solutions of hydrogen peroxide on the market are usually produced by the autoxidation method (the anthraquinone method). Aqueous solutions of hydrogen peroxide produced by this method are not suitable as the above-mentioned washing solutions for use in the electronic industry. To remove impurities in aqueous solutions of hydrogen peroxide, various methods have been proposed.
As a method of purifying an aqueous solution of hydrogen peroxide to high purity, a method has been proposed which comprises removing soluble impurities by passing a crude aqueous solution of hydrogen peroxide through a packed fixed or fluidized bed of a cation exchange resin for removal of cationic impurities, an anion exchange resin for removal of anionic impurities and an adsorbent (such as a synthetic adsorbent or activated carbon) for removal of organic carbon impurities and further removing insoluble impurities through a fine filter (U.S. Pat. No. 4,999,179, and WO92/06918).
Among the organic carbon impurities, dissociable components such as organic acids can be removed mostly by an ion exchange resin, while undissociable organic carbon impurities are removed primarily by various types of adsorbents. However, among impurities contained in an aqueous solution of hydrogen peroxide produced by the autoxidation method, especially the organic carbon impurities include a great variety of components ranging widely from readily adsorbable substances to hardly adsorbable substances in terms of adsorption to an adsorbent. Therefore, when a crude aqueous solution of hydrogen peroxide is passed through an adsorbent, hardly adsorbable organic carbon impurities compete with readily adsorbable organic carbon impurities for adsorption, whereby adsorption of the hardly adsorbable organic carbon impurities will be hindered. Thus, there used to be a problem that the adsorption capacity tends to be too small to produce the desired highly pure aqueous solution.
Therefore, if an adsorbent is used to remove undissociable organic carbon impurities in order to treat the same amount of a crude aqueous solution of hydrogen peroxide as an ion exchange resin can treat to remove dissociable impurities, frequent renewal of the adsorbent or a large amount of the adsorbent is necessary. However, use of a large amount of an adsorbent results in increase in the retention time of an aqueous solution of hydrogen peroxide in an adsorbent device, and when an organic adsorbent is used as the adsorbent, another problem of contamination due to elution of organic impurities from the adsorbent itself arises, whereby the desired highly pure aqueous solution of hydrogen peroxide can not be obtained. On the other hand, when an inorganic adsorbent is used as the adsorbent, hydrogen peroxide decomposes generating oxygen gas appreciably, whereby there is a drawback that stable operation is difficult.